1. Field of the Invention
This invention relates to an aromatic polycarbodiimide, and its sheet. More particularly, the invention relates to an aromatic polycarbodiimide having high resistance to heat, a low dielectric constant, humidity resistance, and flexibility, and a sheet thereof.
2. Description of the Related Art
There have been known aromatic polycarbodiimides produced by polymerizing diphenylmethane diisocyanate (MDI) or tolylene diisocyanate (TDI) as a monomer. These aromatic polycarbodiimides are used as flame resistant films or heat resistant adhesives because of their excellent heat resistance.
These aromatic polycarbodiimide films do not generate a volatile gas or decomposed monomer even when they are exposed to high temperatures above 400xc2x0 C. From this viewpoint, the aromatic polycarbodiimide films are heat resistant. However, they have low humidity resistance, and when heat-treated at 200xc2x0 C. or higher, lose self-supporting properties, and become too brittle to be put to practical use. In addition, they have poor solubility in organic solvents, and low workability.
In recent years, electronic circuits have become compact in size, and light in weight. Along this line, electric wires and cables have been rendered thin, and lightweight. Thus, coating of an electric wire with a thin-walled thermosetting resin is used as very effect means. However, thinning of an electric wire by making the coating thin has its limits, so that thin electric wires are increasingly used.
Among thermosetting resins used as electric wire coating materials are epoxy resins, silicone resins, polyurethane resins, polyamide resins, polyimide resins, and polyesterimide resins. However, coating of a thin electric wire with any of these resins poses the problem that the electric wire is cut during coating, because of the high viscosity of the resin.
To avoid this problem, it is conceivable to lower the viscosity of the resin. For this purpose, the concentration of the resin may be lowered, or the molecular weight of the resin may be made low. However, lowering the concentration of the resin makes it difficult to coat the electric wire to a sufficient thickness. Decreasing the molecular weight of the resin, on the other hand, causes the problem that the coating of the resulting coated electric wire has low strength.
Publicly known polycarbodiimide resins are excellent in heat resistance and humidity resistance in a room temperature atmosphere. However, these resins are extremely unstable in a high pressure, high humidity environment such as in a pressure cooker test. Since the mechanical strength of a film formed from such resins is low, the coating reliability of the electric wire markedly declines.
Japanese Patent Application Laid-Open No. Hei 8-259693 and Japanese Patent Application Laid-Open No. Hei 8-259886 describe that carbodiimide-polyurethane copolymers have excellent heat resistance. However, these copolymers are unsuitable for coating thin electric wires, and they are insufficient in humidity resistance and coating reliability.
The inventors of the present invention have conducted intensive studies on various starting monomers and aromatic carbodiimide polymers to resolve the foregoing drawbacks of the conventional polycarbodiimides. As a result, they have found that the above problems can be solved by a polycarbodiimide having the following new skeleton, and have accomplished the present invention based on this finding.
The present invention has been described in detail with respect to preferred embodiments, and it will now be that changes and modifications may be made without departing from the invention in its broader aspects, and it is the intention, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit of the invention.
An aromatic polycarbodiimide according to the present invention is characterized by having a structural unit represented by the following formula (I): 
where R is an organic group having 3 or more carbon atoms, and n is an integer of 2 to 300.
In the formula, n is preferably an integer of 2 to 100.
In the formula, R is preferably an alkylene group having 3 or more carbon atoms, more preferably 3 to 20 carbon atoms, especially 5 to 10 carbon atoms.
In the formula, R is preferably an alkylidene group having 3 or more carbon atoms.
A polycarbodiimide solution according to the present invention is characterized in that it is prepared by dissolving any one of the above aromatic polycarbodiimides in an organic solvent.
An insulated coated electric wire according to the present invention is characterized by being coated with a resin composition consisting essentially of any one of the aromatic polycarbodiimides.
A polycarbodiimide sheet according to the present invention is characterized by comprising a resin composition consisting essentially of any one of the aromatic polycarbodiimides.